It really depends on lots of things, budget, size, fuel types, cooking styles you do etc.
I have the Rangemaster Toledo which is a modern stainless steel effect one. It cost about £1500 (plus hood and splashback) but I love it. I decided on this model as not many 90cm models have 2 ovens and a sperate grill. I wanted this option, as I often use a grill as well as an oven, therefore didn't want 2 ovens, one being the grill too. Also the feel of some are less solid than others. I wanted cast iron pan rests and a griddle.
Some are duel fuel, eg gas hobs and electric ovens, which I find most practical, but obviously you need to have gas.
Size is really vital, 90cm was all that our new kitchen could take (1 1/2 times the size of a normal oven) but you can get 120cms, which are massive.
Brand names are not necessarily important, however I really feel you get what you pay for, some of the cheaper range cookers are not so robust and are more a fashion style. Britannia are beautiful, but very expensive (£2500), Belling and Stovers are around £1000. I liked Rangemaster as it is owned by the AGA company and has a long heritage of range cookers rather than getting on the band wagon.
I work for an interiors magazine which specialises in Kitchen, so ask me any other questions if you like, I can usually find out 'trade' secrets!! Diplomat for intance is made by Servis!! Hence the value for money.